1. Field of the Invention
The present invention relates generally to a ceramic envelope device used in a high-pressure metal vapor discharge lamp (hereinafter referred to as "HID" lamp; "HID" representing High Intensity Discharge), and a method for producing the ceramic envelope device. More particularly, the invention is concerned with the structure of one or both ends of such a ceramic envelope device at which a translucent ceramic arc tube is closed by an end cap and sealed by a sealing layer, and a method suitable for producing this ceramic envelope device.
2. Discussion of the Prior Art
In the art of such HID lamps using a translucent ceramic arc tube, end caps are inserted in the opposite end portions of the arc tube so as to close the open ends, and support discharge electrodes on their inner surfaces. Such end caps are formed of an electrically conductive cermet material as disclosed in U.S. Pat. Nos. 4,155,757 and 4,155,758, or consist of a laiminar structure wherein an electrically conductive layer is interposed between a plurality of ceramic layers, as disclosed in laid-open Publication No. 62-176043 of unexamined Japanese Patent Application.
While the end caps may be fitted in the corresponding end portions of the ceramic arc tube, by utilizing shrinkage of the arc tube during firing thereof, one of the opposite open ends of the arc tube is generally closed by the appropriate end cap and sealed by a suitable sealing layer after the tube is charged with a suitable metal halide and other substances. The sealing layer is provided to seal a clearance between the end cap and the inner surface of the appropriate end portion of the arc tube. To avoid evaporation or scattering of the metal halide and other contents of the arc tube, the sealing operation is commonly effected by rapidly heating the appropriate end portion of the arc tube and the corresponding end cap to about 1500.degree. C., and by rapidly cooling the heated portion of the tube and the end cap.
In the HID lamp wherein the sealing material is used for sealing at least one of the opposite open ends of the arc tube, the sealing material provided to fill a clearance between the end portion of the tube and the end cap is likely to be exposed on the inner surface of the end cap. The sealing layer is corroded at its exposed portion by the metal halide vapor within the ceramic envelope device, whereby the sealing function of the sealing layer is lowered, causing leakage of the contents of the envelope device, and deteriorated operating characteristics of the lamp, such as lowered luminous efficacy (blackening of the wall of the translucent arc tube), and changes of color and operating voltage. Thus, the exposure of the sealing material to the inner space of the envelope device has adverse influences on the lamp.
To solve the above problem, the applicants proposed a ceramic envelope device for an HID lamp, as disclosed in copending U.S. patent application Ser. No. 07/135,255, filed Dec. 21, 1987, now U.S. Pat. No. 4,808,881, wherein the end portion of the ceramic arc tube which is gas-tightly sealed by a sealing layer is provided with a stepped portion a suitable distance inward of the end face of the tube. The stepped portion has an annular shoulder surface which extends from the inner surface of the tube in the radially inward direction. The outer peripheral portion of the inner surface of the appropriate end cap is held in direct abutting contact with the annular shoulder surface of the arc tube, so that the sealing layer is substantially isolated from the inner space of the envelope device.
In the HID lamp equipped with the thus constructed ceramic envelope device wherein the sealing material is isolated from the inner space, the sealing layer is protected from corrosion by the metal halide vapor, and the resultant drawbacks are effectively eliminated.
However, further study and analysis by the applicants on the above-proposed envelope device showed high possibility of cracking at the end section of the arc tube, in the presence of the stepped portion having a shoulder surface which is defined by two different inside diameters of the different axial portions adjacent to the shoulder surface. Namely, the end portion of the arc tube is likely to have a variation in heat capacity in the axial direction of the tube, due to inconsistency in the wall thickness of the stepped portion in the circumferential direction of the tube, and a difference in the wall thickness or inside diameter between the different axial portions adjacent to the stepped portion. The above variation in the heat capacity may cause cracking of the end section of the arc tube during heat treatment for gas-tight sealing of the appropriate end of the tube with the sealing material In this respect, the proposed ceramic envelope device has some room for further improvement.